Allow mocap bodies to be loaded by the Unity plugin.

PiperOrigin-RevId: 558735711
Change-Id: Ibc9a81cbf8ec4b979d5b325c822c1bffc1408509

unity/Runtime/Components/Bodies/MjMocapBody.cs

@@ -19,7 +19,11 @@ using UnityEngine;
 namespace Mujoco {
   public class MjMocapBody : MjBaseBody {
     protected override void OnParseMjcf(XmlElement mjcf) {
-      throw new Exception("parsing mocap bodies isn't supported.");
+      // Transform
+      transform.localPosition =
+          MjEngineTool.UnityVector3(mjcf.GetVector3Attribute("pos", defaultValue: Vector3.zero));
+      transform.localRotation = MjEngineTool.UnityQuaternion(
+          mjcf.GetQuaternionAttribute("quat", defaultValue: MjEngineTool.MjQuaternionIdentity));
     }
 
     protected override unsafe void OnBindToRuntime(MujocoLib.mjModel_* model, MujocoLib.mjData_* data) {